1. Field of the Invention
The present invention relates to a power transmission device for a hybrid vehicle having an engine, a motor and a power distributor.
2. Description of the Related Art
As a power transmission device for a hybrid vehicle comprising an engine, a motor (electric motor) and a power distributor, there is known a device described in Japanese Patent Laid-Open No. 11-301291 (patent reference 1), for example. The hybrid vehicle comprises an engine, two motors and two power distributors (differential gear units) each comprising a planet gear unit, in which the rotational driving force from the engine is distributed to the input shafts of the power distributors via gears. One of two output shafts of each power distributor is connected to either of the two motors, so that a driving torque (powering torque) or regenerative torque is supplied from the motor to the power distributor. In addition, the other output shafts of the power distributors transmit the rotational driving force to one power output shaft, which is connected to the driving wheels of the vehicle, in parallel. In addition, the speed reduction ratio of the rotation transmission system from the engine to the power output shaft via one power distributor differs from that of the rotation transmission system from the engine to the power output shaft via the other power distributor. Specifically, the speed reduction ratio refers to the speed reduction ratio calculated on the assumption that the rotational speed of the one, connected to the motor, of the two output shafts of the power distributor of the relevant rotation transmission system is 0 (that is, the value obtained by dividing the input rotational speed by the output rotational speed). In the description of prior art, of the speed reduction ratios of the rotation transmission systems, the larger one is referred to as a lower-vehicle-speed speed reduction ratio, and the smaller one is referred to as a higher-vehicle-speed speed reduction ratio.
In the power transmission device for the hybrid vehicle thus configured, if one of the motors is in the driving state (powering state), the other motor is in the regenerative (electric power generating) state, and the power consumed by the driving motor and the power generated by the regenerative motor are kept substantially in equilibrium, the following equation concerning the rotational speed ωe and torque Te of the engine and the rotational speed ωv and torque Tv of the power output shaft holds in the steady state (substantially constant speed); Tv=(ωe/ωv)·Te. Here, the speed reduction ratio (ωe/ωv) from the engine to the power output shaft can be arbitrarily changed between the lower-vehicle-speed speed reduction ratio and the higher-vehicle-speed speed reduction ratio by controlling the torque of the motors.
Thus, when the vehicle runs using the engine as the propulsive source, the torques of the motors can be controlled to vary continuously the speed reduction ratio (speed change ratio) from the engine to the power output shaft, thereby achieving the same performance as a device having an infinite variable speed drive, such as a CVT, provided between the engine and the power output shaft. That is, without any infinite variable speed drive such as CVT, the vehicle can run using the power from the engine while conducting speed variation between the engine and the power output shaft.
In the power transmission device for a hybrid vehicle of this type, some of the energy of the engine is output to the power output side via the regenerative motor and the driving motor in this order. From studies by the inventors, it is shown that, in this case, as the ratio between the lower-vehicle-speed speed reduction ratio and the higher-vehicle-speed speed reduction ratio (lower-vehicle-speed speed reduction ratio/higher-vehicle-speed speed reduction ratio) increases, the part of the output energy of the engine which is distributed to the motors increases, so that the efficiency of energy transmission from the engine to the power output shaft is reduced. In addition, since the energy distributed to the motors increases, the capacities of the motors also increase. Therefore, it is difficult to significantly increase the ratio between the lower-vehicle-speed speed reduction ratio and the higher-vehicle-speed speed reduction ratio in the power transmission device for a hybrid vehicle described in the patent reference 1. As a result, compared with typical cars, the hybrid vehicle often runs at the lower-vehicle-speed speed reduction ratio, and gears or other components of the power transmission device operate at high speeds. In addition, the engine often operates in high-speed revolution regions. Thus, the components of the power transmission device have to be increased in durability, and there is a difficulty in reducing the fuel consumption of the engine.
To eliminate such disadvantages, a speed change unit that changes the lower-vehicle-speed speed reduction ratio or higher-vehicle-speed speed reduction ratio in a plurality of stages may be provided between each power distributor and the power output shaft. However, simply providing such a speed change unit results in increase in size and cost of the power transmission device, which are desired to be reduced to a minimum.
The present invention has been devised in view of such circumstances, and an object of the present invention is to provide a power transmission device that can increase the range of speed variation between an engine and a power output shaft and effectively achieve reduction in size and cost of the device arrangement.